Electron beam focusing device employing a foil wound solenoid



L. T. KING Nov. 12, 1968 ELECTRON BEAM FOCUSING DEVICE EMPLOYING A FOIL WOUND SOLENOID Filed Feb. 21.

3 Sheets-Sheet 1 M a Z W /m 4 .NSNN r a w M Z SK [5 Q\ RvNhuw iv! Eill m h S a p Q\\\ a m N 1 Wm 3 \\\q NQQ Q Nov. 12, 1968 L. T. KING 3,411,033

ELECTRON BEAM FOCUSING DEVICE EMPLOYING A FOIL WOUND SOLENOID Filed Feb. 21, 1967 -3 Sheets-Sheet 2 4x444 p/sr/m/cz [Ava/5) United States Patent 3,411,033 ELECTRON BEAM FOCUSING DEVICE EMPLOY- ING A FOIL WOUND SOLENOID Leonard T. King, Hermosa Beach, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Feb. 21, 1967, Ser. No. 617,622 Claims. (Cl. 315-31) ABSTRACT OF THE DISCLOSURE The disclosed electron beam focusing solenoid includes an electrically conductive foil wound about and connected to a non-magnetic electrically conductive bobbin having a thickness much greater than the foil thickness, with a nonmagnetic electrically conductive tube disposed about and connected to the outermost foil winding. A ferromagnetic tube encircling the foil solenoid is mounted on a pair of ferromagnetic pole pieces extending radially outwardly from opposite ends of the bobbin. The ferromagnetic members provide a magnetic return path and also carry the solenoid input current.

Background of the invention This invention relates generally to magnetic focusing of electron beams, and more particularly relates to a foil wound solenoid arrangement for focusing electron beams of the type used in a traveling-wave tube.

In traveling-wave tubes a stream of electrons is caused to interact with an electromagnetic wave propagated along a slow-wave structure disposed along and about the electron stream path. In order to achieve high operating efliciency and long life for the tube, electron impingement on the slow-wave structure must be minimized. Thus, the electrons must be focused into a well-collimated precisely constrained beam traveling axially along the slow-wave structure. Such focusing has been accomplished by immerising the electron stream in a strong axial magnetic field generated by either one or more permanent magnets or solenoids.

Prior art focusing solenoids have usually consisted of a continuous winding of round or square wire or a set of series connected segments of wound conductive foil. For wire solenoids, the finite pitch angle of the wire winding introduces a transverse magnetic field on the solenoid axis. With a series of foil segments, interconnections between the foil segments generate transverse magnetic field components, and the axial separation of the segments introduces variations in the axial magnetic field. Moreover, the flow of input and output current for both wire and segmented foil solenoids generates significant transverse magnetic field components on the solenoid axis.

Summary of the invention Thus, it is an object of the present invention to provide a magnetic device for focusing an electron beam which provides a reduced ratio of transverse magnetic field to axial magnetic field along the axis of the device.

Accordingly, the electron beam focusing device of the invention includes an inner non-magnetic electrically conductive tube and an electrically conductive tape having a width slightly less than the length of the tube wound about the tube, the innermost tape winding being electrically connected to the tube. An electrically insulating tape is also wound about the tube between successive windings of the conductive tape, and an outer non-magnetic electrically conductive tube is disposed about and is electrically connected to the outermost winding of the conductive tape. A pair of ferromagnetic annular disks ice are mounted on and are electrically connected to respective opposite ends of the inner tube, with the disks extending radially outwardly to a distance beyond the outer non-magnetic tube. A ferromagnetic tube, which is spaced from and which encircles the outer non-magnetic tube, is mounted on and is electrically connected to each of the disks. An electrical potential is applied between the outer non-magnetic tube and the ferromagnetic tube.

This arrangement ensures that current is uniformly distributed to and removed from a single wound tape, while at the same time transverse components of input and output current are caused to substantially cancel one another, thereby minimizing the transverse magnetic field along the axis of the arrangement.

Brief description of the drawing In the accompanying drawing:

FIG. 1 is a longitudinal cross-sectional view, partly in schematic form, of an electron beam focusing device in accordance with one embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along line 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary cross-sectional view taken along line 33 of FIG. 1;

FIG. 4 is a graph illustrating the ratio of transverse magnetic field to axial magnetic field as a function of axial distance from the electron gun end of the device of FIGS. l3;

FIG. 5 is a longitudinal sectional View, partly in schematic form, of an electron beam focusing device in accordance with another embodiment of the invention; and

FIG. 6 is a cross-sectional view taken along line 66 of FIG. 5.

Description 0 the preferred embodiments Referring to FIG. 1 with greater particularity, reference numeral 10 designates an electron gun, which may be of a conventional construction well-known in the art, which functions to launch a stream of electrons along a slow-wave structure 12 in the direction of a collector electrode 14. A focusing arrangement designated generally by numeral 16 and constructed in accordance with the principles of the invention is disposed about the slowwave structure 12 in order to focus the traveling electrons into an extremely well-collimated beam 18.

The focusing arrangement 16 includes a single solenoid 20 extending substantially throughout the length of the slow-wave structure 12. The solenoid 20 includes a tubular core, or bobbin, 22 of a non-magnetic electrically conductive material which also has a high thermal conductivity. Examples of suitable materials which may be employed for the bobbin 22 are copper, silver, and aluminum, although cooper is preferred on account of its better thermal properties.

As may be best seen from FIG. 3, the windings of the solenoid 20 consist of an electrically conductive tape 24 which is wound about the bobbin 22. The tape 24 may be a copper foil having a thickness of around 4 mils, for example, and a width slightly less than the length of the bobbin 22. The innermost winding of the electrically conductive tape 24 is connected to the outer surface of the bobbin 22, for example by brazing. Moreover, in order to insure a sufiiciently low impedance (short circuit) connection to the innermost solenoid turn, the bobbin 22 should have a thickness at least an order of magnitude greater than that of the tape 24. For the aforementioned exemplary tape thickness of 4 mils, a typical bobbin thickness might be mils. An electrically insulating tape 26, such as l-mil thick paper for example, is also wound about the bobbin 22 between successive windings of the conductive tape 24. The number of turns of conductive tape 24 and insulating tape 26 may be several hundred, for example.

A second short-circuiting tubular member 28 of a nonmagnetic electrically conductive material is disposed about and is electrically connected to the electrically conductive tape 24. The outer tubular member 28 may be of any of the materials set forth above for the bobbin 22, copper being preferred however. Also, the member 28 should have a thickness substantially greater than that of the electrically conductive tape 24, although the thickness of the outer tube 28 need not be as great as that of the bobbin 22.

A pair of disk-like annular pole pieces 30 and 32 of ferromagnetic material are disposed at the respective ends of the bobbin 22 and are secured to the bobbin 22, by brazing for example, so that good electrical connection is afforded between the pole pieces 30 and 32 and the bobbin 22. The pole pieces 30 and 32 extend radially outwardly from the bobbin 22 to a distance beyond the nonmagnetic tubular member 28. A tubular member 34 of ferromagnetic material is mounted on and is secured to the outer circumferential surfaces of the pole pieces 30 and 32, by brazing for example, so as to provide good electrical connection with the pole pieces 30 and 32. The ferromagnetic members 30, 32 and 34 not only provide a return path for the magnetic field generated by the solenoid axially along the electron beam 18, but these members also function to convey input electrical current for the solenoid to the bobbin 22.

An electrically conductive ring 36 is attached, for example by brazing, to the outer circumferential surface of the ferromagnetic tubular member 34 to afford ready electrical connection thereto, as well as to increase the circumferential electrical conductivity of the magnetic circuit including members 30, 32 and 34. An electrical lead wire 38 is, in turn, connected to the ring 36. A pair of electrical lead wires 42 and 44, are connected to diametrically opposite points on the outer circumferential surface of the non-magnetic tubular member 28 and extend radially outwardly therefrom through respective diametrically opposite holes 46 and 48 in the ferromagnetic tubular member 34. The leads 42 and 44 are connected together externally of the device 1'6 and are also connected to one terminal of a power supply 50, the other terminal of the power supply being connected to the lead 38.

It is pointed out that only one electrical connection is needed to the ring 36, because the shielding effect of ferromagnetic tubular member 34 prevents current flow through the lead 38 from establishing a transverse magnetic field in the region adjacent the electron beam 18. On the other hand, current flow through the portions of the leads 42 and 44 which extend inwardly of the member 34 do generate transverse magnetic fields in the vi cinity of the electron beam; however, these fields are equal and opposite, and hence cancel one another. It is further pointed out that it is not necessary to employ two diametrically opposite leads in order to obtain a transverse field canceling effect, but rather any practical plurality of equally circumferentially spaced radially extending leads may be used.

Current flows from the power supply 50 through the lead 38, the connecting ring 36, the ferromagnetic member 34, and the pole pieces and 32 to the solenoid bobbin 22. Current then flows spirally outwardly through the solenoid foil winding 26 to the outer non-magnetic tubular member 28, thereby generating an axial magnetic field along the electron beam 18. The solenoid current returns to the power supply via leads 42 and 44.

Since the focusing arrangement of the present invention is constructed in a single section extending essentially throughout the entire length of the focusing device, a highly runiform axial magnetic field is provided. Moreover, since no interconnections between a plurality of wound foil sections are needed, transverse magnetic field components introduced by such interconnections are eliminated. In addition, the uniform radial flow of solenoid input current through the pole pieces 30 and 32 produces canceling transverse magnetic fields and thereby further reduces transverse magnetic field components in the vicinity of the electron beam. Also, the canceling effect of radially oppositely flowing solenoid output output current through the leads 42 and 44 affords a still further reduction in transverse magnetic field.

By employing a bobbin 22 having a thickness substantially greater than the foil thickness, current from the pole pieces 30 and 32 is distributed as uniformly as possible to the first turn of the solenoid. Also, the bobbin 22 functions as a short-circuited solenoid turn to minimize any alternating magnetic field components caused by alternating current components generated by the solenoid power supply.

The focusing arrangement of the present invention is thus able to provide an extremely uniform axial magnetic focusing field with a vastly reduced magnitude of transverse, or radial, magnetic field components along the axis of the arrangement. This reduction in transverse magnitude field may be appreciated by referring to the graph of FIG. 4 in which the ratio of transverse magnetic field to axial magnetic field is shown as a function of distance along the axis of a focusing device constructed in accordance with FIGS. 1-3. It may be seen from FIG. 4 that the ratio of transverse magnetic field to axial magnetic field is less than .083% throughout almost the entire length of the device and is as low as .017% throughout a substantial portion of the length of the device. On the other hand, the lowest ratio of transverse magnetic field to axial magnetic field generally achievable with prior art focusing devices is .17%. Thus, the present invention is able to reduce the achieveable ratio of transverse magnetic field to axial magnetic field an order of magnitude below that of the prior art.

An alternate embodiment of the present invention is shown in FIGS. 5 and 6. The embodiment of FIGS. 5-6 is similar to that of FIGS. 1-3, and hence respective elements in the FIGS. 5-6 embodiment are designated by the same reference numerals as their corresponding elements in the embodiment of FIGS. 1-3 except for the addition of a prefix numeral 1. Moreover, in the embodiment of FIGS. 5-6 electrically conductive ring 136 which is secured to the outer circumferential surface of ferromagnetic tubular member 134 is located at the collector end of the assembly 116 in alignment with the pole piece 132. This location better assures the desired increase in the circumferential electrical conductivity of the magnetic circuit.

In addition, in the embodiment of FIGS. 5-6 four coaxial transmission line leads 151, 152, 153 and 154 are provided to the foil wound solenoid 120. Respective inner conductors 161, 162, 163 and 164 of the coaxial lines 151, 152, 153 and 154 are connected to equally circumferentially spaced points along the outer circumference of outer non-magnetic tubular member 28 at a longitudinal location near the center of the member 128. The coaxial lines 151, 152, 153 and 154 are disposed parallel to the electron beam 118 in the space between the nonmagnetic member 128 and the ferromagnetic member 134 and extend outwardly from the assembly 116 through respective equally circumferentially spaced holes in the collector pole piece 132. Respective outdoor conductors 171, 172, 173 and 174 for the coaxial lines 151, 152, 153 and 154 are attached, by soldering for example, to the pole piece 132 in order to afford electrical connection to the solenoid bobbin 122. Each of the inner coaxial conductors 161-164 is connected to one terminal of power supply 150, while each of the outer coaxial conductors 171174 is connected to the other terminal of the power supply 250.

Although the present invention has been shown and described with reference to specific embodiments, nevertheless various changes and modifications obvious to a person skilled in the art to which the invention pertains are deemed to lie within the purview of the invention.

What is claimed is:

1. An electron beam focusing device comprising: a first non-magnetic electrically conductive tubular member, an electrically conductive tape having a width slightly less than the length of said first tubular member wound about said first tubular member, the innermost winding of said tape being electrically connected to said first tubular member, an electrically insulating tape wound about said first tubular member between successive windings of said conductive tape, a second non-magnetic electrically conductive tubular member disposed about and being electrically connected to the outermost winding of said electrically conductive tape, first and second ferromagnetic annular disks mounted on and electrically connected to respective ends of said first tubular member and extending radially outwardly beyond said second tubular member, a ferromagnetic tubular member mounted on and electrically connected to said disks, said ferromagnetic tubular member being spaced from and encircling said second tubular member, and means for applying an electrical potential between said second tubular member and said ferromagnetic tubular member.

2. An electron beam focusing device according to claim 1 wherein the thickness of said electrically conductive tape is substantially less than the thickness of said second tubular member and is at least an order of magnitude less than the thickness of said first tubular member.

3. An electron beam focusing device according to claim 1 wherein said ferromagnetic tubular member defines a plurality of equally circumferentially spaced radially extending holes at a predetermined longitudinal location along said ferromagnetic tubular member, and wherein said means for applying an electrical potential includes: a power supply, a plurality of electrical leads passing through respective ones of said holes and extending radially inwardly to respective equally circumferentially spaced points on the outer surface of said second tubular member, each of said leads bein electrically connected between one of said points and a terminal of said power supply, an electrically conductive ring mounted on and electrically connected to the outer circumferential surface of said ferromagnetic tubular mem her, and an electrical lead connected between said ring and another terminal of said power supply.

4. An electron beam focusing device according to claim 1 wherein one of said annular disks defines a plurality of equally circumferentially spaced longitudinally extending holes at locations radially between said second tubular member and said ferromagnetic tubular member, and wherein said means for applying an electrical potential includes: a power supply, a plurality of coaxial transmission lines each having an inner conductor and an outer conductor passing through respective ones of said holes and extending into the space between said second tubular member and said ferromagnetic tubular member, respective inner conductors of said coaxial transmission lines being electrically connected between respective equally circumferentially spaced points on the outer surface of said second tubular member at a predetermined location along said second tubular member and a terminal of said power supply, and the outer conductor of each of said coaxial transmission lines being electrically connected to said one annular disk and to another terminal of said power supply.

5. An electron beam focusing device according to claim 4 wherein an electrically conducting ring is mounted on and electrically connected to the outer circumferential surface of said ferromagnetic tubular member adjacent said one annular disk.

References Cited UNITED STATES PATENTS 2,624,859 1/1953 Smullin et al 3l53.5 3,243,639 3/1966 GlaSS 33521O X 3,334,264 8/1967 Niclas 3l53l X RICHARD A. FARLEY, Primary Examiner.

H. C. WAMSLEY, Assistant Examiner. 

